Inorganic Chemistry, Vol.59, No.5, 2689-2700, 2020
Effect of Lewis Acids on the Structure and Reactivity of a Mononuclear Hydroxomanganese(III) Complex
The addition of Sc(OTf)(3) and Al(OTf)(3) to the mononuclear Mn-III-hydroxo complex [Mn-III (OH)(dpaq)](+) (1) gives rise to new intermediates with spectroscopic properties and chemical reactivity distinct from those of [Mn-III (OH)(dpaq)](+). The electronic absorption spectra of [Mn-III (OH)(dpaq)](+) in the presence of Sc(OTf)(3) (1-Sc-III) and Al(OTf)(3) (1-Al-III) show modest perturbations in electronic transition energies, consistent with moderate changes in the Mn m geometry. A comparison of H-1 NMR data for 1 and 1-Sc-III confirm this conclusion, as the H-1 NMR spectrum of 1-Sc-III shows the same number of hyperfine-shifted peaks as the H-1 NMR spectrum of 1-Al-III, These H-1 NMR spectra, and that of 1-Al-III, share a similar chemical-shift pattern, providing firm evidence that these Lewis acids do not cause gross distortions to the structure of 1. Mn K-edge X-ray absorption data for 1-Sc-III provide evidence of elongation of the axial Mn-OH and Mn-N(amide) bonds relative to those of 1. In contrast to these modest spectroscopic perturbations, 1-Sc-III and 1-Al-III show greatly enhanced reactivity toward hydrocarbons. While 1 is unreactive toward 9,10-dihydroanthracene (DHA), 1-Sc-III and 1-Al-III react rapidly with DHA (k(2) = 0.16(1) and 0.25(2) M-1 s(-1) at 50 degrees C, respectively). The 1-Sc-III species is capable of attacking the much stronger C-H bond of ethylbenzene. The basis for these perturbations to the spectroscopic properties and reactivity of 1 in the presence of these Lewis acids was elucidated by comparing properties of 1-Sc-III and 1-Al-III with the recently reported Mn-III -aqua complex [Mn-III (OH2)(dpaq)](2+) (J. Am. Chem. Soc. 2018, 140, 12695-12699). Because 1-Sc-III and 1-Al-III show H-1 NMR spectra essentially identical to that of [Mn-III(OH2)(dpaq)](2+), the primary effect of these Lewis acids on 1 is protonation of the hydroxo ligand caused by an increase in the Bronsted acidity of the solution.